Toward the crystal structure of nagyagite, [Pb(Pb,Sb)S2][(Au,Te)

Abstract

Synthetic nagyagite was grown from a melt as part of a search for materials with high-temperature superconductivity. Electron microprobe analyses of synthetic nagyagite and of nagyagite from the type locality Nagyág, Transylvania (now Săcărîmb, Romania) agree with data from literature. The crystal chemical formula [Pb(Pb,Sb)S 2 ][(Au,Te)] was derived from crystal structure investigations. Nagyagite is monoclinic pseudotetragonal. The average crystal structure was determined from both synthetic and natural samples and was refined from the synthetic material to R = 0.045 for 657 single-crystal X-ray data: space group P2 1 /m, a = 4.220(1) Å, b = 4.176(1) Å, c = 15.119(3) Å, β = 95.42(3)°, and Z = 2. Nagyagite features a pronounced layer structure: slices of a two slabs thick SnS-archetype with formula Pb(Pb,Sb)S 2 parallel to (001) have a thickness of 9.15 Å. Te and Au form a planar pseudo-square net that is sandwiched between the SnS-archetype layers; it is assumed that planar Au [4Te] Te 4 configurations are edge connected to chains and that Te atoms are in a zigzag arrangement. Ordering within the SnS-archetype and gold-tellurium layers, intense twinning and/or stacking variants are responsible for the often observed superstructure reflections. For buckhornite, [(Pb 2 Bi) ∑3 S 3 ][(AuTe 2 ) ∑3 ], a structure model is proposed considering a homologous series with nagyagite, [(Pb 3 (Pb,Sb) 3 ) ∑6 S 6 ][(Au,Te) 3 ].